I think you may need to write your own assignment handler to handle that. However, your definition is not good enough because you do not specify the relationship between two assignment.If the relationship is OR and you are using the identity model JBPM provides, you can create a parent group which include these 2 child groups in database.

The Group class has a method addChild. you can create subgroup by that. It requires the knowledge of hibernate. I am not using the JBPM identity model so I won't be able to give detail information for that.